Present invention relates to a color image forming method and apparatus and, more particularly, to a color image forming method and apparatus for forming a full-color image based on image data having a mixture of different types of data, such as character data, drawing data and picture data, by sequentially overlaying a plurality of color images formed on an electrostatic drum onto a transfer member.
Recently, color printers have been put to practical use as various expression means for users. Especially, a color page printer of electrophotographic printing method has attracted public attention because of its silent operation, high image quality and high-speed printing.
One of those color page printers, a full-color laser-beam printer performs development using a first toner, by scanning a laser beam on an electrostatic member in a main-scanning direction, and transfers the developed image onto a recording medium such as a recording sheet. The printer performs the above operation as a first step, then performs second to fourth steps using second to fourth toners, thus performs full-color image formation and printing.
More specifically, the color laser-beam printer of electrophotographic printing method forms toner images in the above-described four steps using four toners, Y (yellow), M (magenta), C (cyan) and K (black), and overlay-transfers the images on a recording medium to obtain a color image. Usually, this type of printer prints character images at 400 dpi (dot per inch) printing density, and as to picture images, prints each pixel of picture image (200 dpi) with two pixels of 400 dpi image in the main-scanning direction.
Especially, a printer recently introduced for practical use is a printer that receives a half-tone image data comprising an 8-bit/pixel multi-level image signal, which has not been binarized by a binarization method such as the dither method, from a host computer (hereinafter referred to as "host"), performs pulse-width modulation on the respective pixels, and outputs a multi-level image.
Next, the conventional recording method by this full-color printer will be described with reference to FIGS. 36 and 37. FIG. 36 is a cross-sectional view showing the construction of the conventional full-color printer. FIG. 37 is a block diagram showing the flows of various signals used by the printer in FIG. 36.
In FIG. 36, an electrostatic drum 201, which rotates at a fixed speed in a direction represented by the arrow, is charged by an electrostatic charger 204 to a predetermined voltage and a predetermined polarity. Next, a recording sheet P is supplied from a paper cassette 215 by a paper feeding roller 214, at predetermined timing, one sheet at a time. When a detector 202 detects the front end of the recording sheet P, a semiconductor laser 205 emits laser light L, modulated by an image signal VDO (8-bit/pixel/color), to a polygon mirror 207. The laser light L, reflected by the polygon mirror 207, and guided onto the electrostatic drum 201 via a lens 208 and a mirror 209, sweeps on the electrostatic drum 201. On the other hand, a signal from the detector 202 (hereinafter referred to as "TOPSNS") is outputted as a vertical synchronizing signal to an image forming unit 250 as shown in FIG. 37. When a detector 217 detects the laser light L, it outputs a beam-detect signal (hereinafter abbreviated to as "BD signal"), which is a horizontal synchronizing signal, to the image forming unit 250. The image signal VDO is sequentially transmitted to the semiconductor laser 205 in synchronization with the BD signal.
The scanner motor 206 rotates at a fixed speed in accordance with a signal S2 from a frequency divider 221, which changes the period of a signal S1 from a reference oscillator 220 under the control of a motor controller 225.
The electrostatic drum 201 is scan-exposed in synchronization with the BD signal, then a first electrostatic latent image is developed by a developer 203Y having yellow toner, and a yellow toner image is formed on the electrostatic drum 201.
On the other hand, immediately before the front end of the recording sheet P reaches a transfer start position, a predetermined transfer bias voltage of an opposite polarity to that of the toner is applied to the transfer drum 216. The yellow toner image is transferred onto the recording sheet P, and at the same time, the recording sheet P is electrostatically attached to the transfer drum 216.
Next, a second electrostatic latent image is formed on the electrostatic drum 201 by the scanning of the laser light L, then the second latent image is developed by a developer 203M having magenta toner. The position of the magenta toner image on the electrostatic drum 201 is aligned, by the TOPSNS signal, with the position of the first (yellow) toner image, and the second toner image is transferred onto the recording sheet P.
In a similar manner, a third electrostatic latent image is developed by a developer 203C having cyan toner, then the position of the cyan toner image is aligned with that of the previous magenta image, and the third toner image is transferred onto the recording sheet P. Finally, a fourth electrostatic latent image is developed by a developer 203 K having black toner, the position of the black toner image is aligned with that of the previous cyan image, and the fourth toner image is transferred onto the recording sheet P.
In each step, a VDO signal for one page is sequentially outputted to the semiconductor laser 205. After each transfer, untransferred toner on the electrostatic drum 201 is scraped off by a cleaner 210.
Thereafter, as the front end of the recording sheet P, on which the four toner images have been transferred, approaches a separation claw 212, the separation claw 212 moves to contact the surface of the transfer drum 216 so as to separate the recording sheet P from the transfer drum 216. The separation claw 212 is in contact with the transfer drum 216 till the rear end of the recording sheet P is separated from the transfer drum 216, thereafter, returns to the initial position. An electrostatic discharger 211 removes the accumulated charge on the recording sheet P, thus assists separation of the recording sheet P by the separation claw 212, and reduces discharge in the air upon separation of recording sheet.
The finally developed image on the recording sheet P is fixed by a fixing roller 213 and the recording sheet P is discharged to a tray 229.
It should be noted that the image forming unit 250 in FIG. 37 is a generic term of all the elements in FIG. 36 excluding the semiconductor laser 205, the scanner motor 206, the polygon mirror 207, and the detectors 202 and 217.
FIG. 38 is a timing chart showing the relation between the TOPSNS signal and the VD signal. In FIG. 38, term A1 corresponds to the first toner color printing; A2, printing of the second toner color printing; A3, the third toner color printing; and A4, the fourth toner color printing. The color printing for one page is from the term A1 to the term A4.
Next, image signal processing will be described.
FIG. 39 is a block diagram showing the functional construction of a conventional full-color printer 302. In FIG. 39, a host interface 303 receives print information 307 from an external device, e.g., a host computer 301, transmits a control signal 308 included in the received print information 307 to a printer controller 304, and also transmits an image signal 309 included in the print information 307 to an image processor 305. The image processor 305 outputs a signal to drive a semiconductor laser 306. The printer controller 304 controls the image processor 305 by a control signal 310.
FIG. 40 is a block diagram showing the detailed construction of the image processor 305 shown in FIG. 39. In FIG. 40, a color processor 351 receives a 24-bit RGB image signal from the host interface 303 shown in FIG. 39, and sequentially converts the input RGB signal into a corresponding YMCK signal at predetermined timing. That is, the color processor 351 converts an input RGB signal, to an 8-bit VDO signal indicative of a Y signal on one occasion; to an 8-bit VDO signal indicative of a M signal on another occasion; to an 8-bit VDO signal indicative of a C signal on till another occasion; and an 8-bit VDO signal indicative of a K signal on till another occasion.
FIG. 41 is a timing chart showing the color signal conversion by the color processor 351. In FIG. 41, terms A1 to A4 represent the respective toner color printing operations, as described in FIG. 38. As indicated by numerals R1, G1 and B1, the same color signal is used in the corresponding toner color printing operation. The 2-bit color designation signal indicates the color of each printing operation. In the color designation signal, numeral "B" added to the respective values indicates that the values are in binary representation.
The YMCK VDO signal from the color processor 351 is .gamma.-corrected by a .gamma. corrector 352, and outputted as an 8-bit signal to a pulse-width modulator (hereinafter abbreviated to "PWM") 353. The PWM 353 latches the input 8-bit image signal by the latch 354 in synchronization with the rising edge of an image clock (VCLK), then converts the latched digital data into analog data by a D/A converter 355 and outputs the data into an analog comparator 356.
On the other hand, the image clock (iVCLK) also enters to a triangular wave generator 358, which converts the image clock into a triangular wave and outputs it to the analog comparator 356.
The analog comparator 356 compares the triangular wave from the triangular wave generator 358 and the analog signal from the D/A converter 355, and outputs a pulse-width modulated signal. This signal is inverted by an inverter 357, thus a PWM signal is obtained.
When the value of an 8-bit image data inputted into the PWM 353 is the maximum "FF(H)", a PWM having the widest pulse width is outputted, while when the value of the 8-bit image data is the minimum "00(H)", a PWM having the narrowest pulse width is outputted.
However, in the conventional signal processing, compared with toner of a conventional monochromatic laser beam printer, color toner (Y, M, C, K colors) is expensive, further, one image is printed by four color toners. Thus, high toner costs and high printing cost per one image are not negligible.
In addition, the conventional color printing has not produced a printer which actually has high resolution (e.g., 600 dot per inch (dpi)) for outputting character images and picture images with satisfactorily high quality yet, since a printer having such high-resolution, which has been desired for many years, possesses the following problems:
(1) For example, when a high-resolution color printer having 600 dpi printing density is manufactured, if 8-bit/pixel multi-level bit data (picture image) is used, such 600 dpi output cannot obtain sufficient tonality due to diameter of toner particle or some processing factor. PA1 (2) Upon outputting a one-page image having a mixture of character image and picture image, a signal indicative of output attribute, i.e. resolution-oriented output for character image or tonality-oriented output for picture image, is necessary. The data structure in this case consists of nine bits, i.e., 8-bit multi-level data and one control bit. PA1 (a) wastes memory; PA1 (b) lowers data processing efficiency; PA1 (c) hardly accepts standardized coding techniques such as data compressing; PA1 (d) causes complexity of process to define one-bit attribution designation signal. PA1 (3) The conventional laser color printer operates to always output images at the maximum density. For example, even upon test printing to see an image layout, the toner is used as in a normal output, and such toner consumption raises running cost.
Accordingly, 600 dpi printing density output is suitable to outputting of image (resolution-oriented image) where resolution is more important than tonality such as characters and drawings, whereas not suitable to outputting of image (tonality-oriented image) where tonality is more important than resolution such as a natural picture.
In order to solve this problem, as to outputting picture images, printing each pixel of picture image (300 dpi) with two pixels of 600 dpi image in the main-scanning direction has been conventionally used. In this method, the picture image is represented in 300 dpi. Compared with the 600 dpi printing, tonality is improved, however, to attain fully improved tonality in representation of 8-bit/pixel input data is still difficult in the conventional process.
However, as the bus width of a data bus normally used in a host or a printer controller is an integral multiple of eight (8 bits, 16 bits, 32 bits etc.). The 9-bit/pixel data:
To solve this problem, the look-up table (hereinafter abbreviated to "LUT") for .gamma. correction of the printer may be changed to lower output density, however, this disturbs the balance among .gamma. characteristics with respect to Y, M, C and K toners, which changes overall tonality in an output image.
The image data may be converted to data for outputting at a lower density by an application soft in the host. However, the density changing must be performed over the whole image data while maintaining the overall tonality of the output image. The problem is that data process time is prolonged for this purpose.